Sunscreen for a Day Outdoors Calculator
Estimate how much sunscreen a full day outdoors uses, including reapplication, and whether a bottle will last.
How it works
applications = floor(hours ÷ reapply interval) + 1 · total = applications × grams per application (by exposed area)
Dermatologists recommend about one ounce — a shot-glass, roughly 30 g — to cover an adult’s exposed body, reapplied every two hours and after swimming or heavy sweating. Most people apply far less than that, which is why real SPF protection falls short. This calculator counts the initial application plus each reapplication over your day and scales the per-use amount by how much skin is exposed, then checks how many days your bottle covers.
Worked example
Six hours reapplying every 2 hours is floor(6 ÷ 2) + 1 = 4 applications. In shorts and a t-shirt (~18 g each) that is 72 g ≈ 2.5 oz, so a 6-oz bottle lasts about 2.4 days.
Frequently asked questions
How much sunscreen should I use?
About one ounce (a shot glass, ~30 g) to cover the whole exposed body, or roughly a teaspoon for the face and neck. Under-applying is the most common reason sunscreen underperforms its SPF.
How often should I reapply?
Every two hours as a rule, and immediately after swimming, toweling off or heavy sweating — even with water-resistant formulas.
Does a higher SPF mean I can use less?
No. SPF is measured at a set application thickness, so a high number does not offset using too little. Apply the full amount and reapply regardless of SPF.
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This calculator is for educational purposes. Double-check important results before acting on them.